Edexcel Geography A-level: Tectonic Processes and Hazards Detailed Study Notes

Global Distribution of Tectonic Hazards

• Hazard Definition: A hazard is defined as a potential threat to human life and property.
• Natural Hazard Categories:     * Hydro-meteorological Hazards: These are caused by climatic processes.     * Geophysical Hazards: These are caused by land processes.
• Spatial Distribution:     * Geophysical hazards predominantly occur near plate boundaries. The movement of these plates at varying speeds and directions results in collisions, earthquakes, and volcanic activity.     * Intra-plate Earthquakes: Earthquakes can also occur in the middle of tectonic plates. While the causes are not fully understood, it is theorized that pre-existing weaknesses in the plates become reactivated. This can result in seismic waves if solid crust, weakened over time, cracks under intense pressure.     * Volcanic Hotspots: Localized areas of the lithosphere (comprising the Earth’s crust and upper mantle) with unusually high temperatures due to the upwelling of hot molten material from the core.         * This theory was first proposed by Tuzo Wilson in $1963$.         * Example: The Hawaii hotspot involves magma rising as a plume of hot rock.         * Ring of Fire: A noted concentrated area of hotspots and boundaries.
• Seismic Belts:     * OFZ (Oceanic Fracture Zone): A belt of activity running through the oceans, along mid-ocean ridges, Africa, the Red Sea, and the Dead Sea.     * CFZ (Continental Fracture Zone): A belt of activity along mountain ranges extending from Spain through the Alps, the Middle East, and to the Himalayas.
• Boundary Intensity: Generally, the most powerful earthquakes occur at convergent or conservative boundaries.

Tectonic Trends and Impact Reporting Since $1960$

• Observed Trends:     * The total number of recorded hazards has increased.     * Overall fatalities have decreased, though significant spikes occur during "mega-disasters."     * The total number of people affected is increasing, driven by global population growth.     * Economic costs have increased significantly. This is attributed to higher levels of development; infrastructure in developed nations is more expensive to repair, and an increase in the number of insurance policies (particularly in developed regions) heightens reported costs.
• Challenges in Reporting Disaster Impacts:     * Direct vs. Indirect Fatalities: Data depends on whether researchers count immediate deaths or those caused later by diseases spreading post-disaster. Some impacts take considerable time to become apparent.     * Location Constraints: Rural and isolated areas are difficult to reach for data collection. Conversely, extremely high population densities also make accurate counting difficult.     * Methodological Variance: Different organizations use different data collection methods, leading to conflicting numbers for deaths and injuries.     * Political Bias: Governments may manipulate data.         * Example: After the $2004$ Indian Ocean tsunami, the Burmese government claimed there were $0$ deaths in Burma, likely to project an image of effective governance and protection.

Earth’s Internal Structure and Tectonic Theory

• The Four Sections of Earth:     1. Crust (Lithosphere): The uppermost, thinnest, least dense, and lightest layer.         * Oceanic Crust: Approximately $7\,km$ thick.         * Continental Crust: Can be up to $70\,km$ thick.     2. Mantle (Asthenosphere): Located at depths from $700\,km$ to $2890\,km$ below the crust. It is composed largely of silicate rocks rich in iron and magnesium. It is semi-molten. A temperature gradient toward the core generates convection currents, which circulate the mantle and contribute to plate movement.     3. Outer Core: Dense, semi-molten rocks containing alloys of iron and nickel. Found at depths between $2890\,km$ and $5150\,km$ below the surface.     4. Inner Core: Solid due to extreme pressures, with a similar composition to the outer core. It is located over $5150\,km$ below the crust.
• Core Heat Sources:     * Primordial Heat: Left over from the Earth's initial formation.     * Radiogenic Heat: Produced through the process of radioactive decay.

Plate Boundaries and Geomorphology

Destructive Plate Boundaries

• Continental and Oceanic Interaction:     * The denser oceanic plate subducts beneath the continental plate.     * This creates a deep ocean trench.     * Oceanic crust melts in the asthenosphere, and the resulting extra magma builds pressure.     * Magma forces through weaknesses in the continental plate, creating explosive, high-pressure composite volcanoes.     * Fold Mountains: Formed when sediment is pushed upward during the subduction process.
• Oceanic and Oceanic Interaction:     * The heavier plate subducts, forming an ocean trench and fold mountains.     * Underwater volcanoes burst through the oceanic plate due to pressure.     * Cooled lava creates new land structures known as island arcs.
• Continental and Continental Interaction:     * Both plates have low density, meaning no subduction of continental crust occurs (though ancient oceanic crust may subduct slightly).     * High pressure causes the continental crust to pile up on top of the lithosphere, forming fold mountains.

Constructive Plate Boundaries

• Oceanic to Oceanic:     * Magma rises into the gap created as plates separate, cooling to form new land.     * Sea Floor Spreading: The process where the ocean floor gets wider as lava fills gaps. Theorized by Harry Hess in the $1940s$.     * Creates less explosive underwater volcanoes.
• Continental to Continental:     * Separation forces land apart, creating a rift valley.     * Horsts: The lifted areas of rock.     * Graben: The valley floor itself.     * Magma rises to form volcanoes; eventually, the gap may fill with water, separating landmasses.

Conservative Plate Boundaries

• Parallel plates move in different directions or at different speeds.
• No plates are destroyed; no new landforms are created.
• High friction builds pressure. On oceanic crust, this can displace huge volumes of water. On continental crust, it causes fault lines and ground cracking.

Mechanisms and Evidence of Plate Movement

• Types of Crust:     * Oceanic: High density, mainly basalt, thin, and young.     * Continental: Low density, mainly granite, thick, and old.
• Movement Mechanisms:     * Mantle Convection: Thermal energy from radioactive decay in the core heats the lower mantle, causing it to rise. As it cools and becomes denser, it sinks. These convection currents push the plates.     * Slab Pull: First theorized by Dan McKenzie. The most dense, old oceanic crust submerges into the mantle, dragging the rest of the plate with it. Researchers now believe this is the primary mechanism as convection currents are often considered too weak to move massively dense plates.     * Ridge Push: Also known as gravitational sliding. Gravity acts on the higher elevation of the mid-ocean ridge, pushing plates further apart and widening the gap.
• Evidence: Paleomagnetism involves the study of magnetic grains in cooling rock aligning with Earth's poles. Because the magnetic poles switch periodically, geologists observe symmetrical, alternating bands of magnetic polarity on either side of constructive boundaries.

Earthquake Dynamics and Seismic Waves

• The Process: Plates become stuck due to friction. Convection currents continue to push, building pressure until the plates give way. This sudden release causes a jolt, radiating seismic (shock) waves.
• Focus (Hypocentre): The underground point of origin.
• Epicentre: The surface point directly above the focus.
• Categories of Seismic Waves:     * Primary (P) Waves: Compressional; vibrate in the direction of travel; travel through solids; speed: $4-8\,km/s$.     * Secondary (S) Waves: Vibrate at right angles to travel; travel only through solid rocks; speed: $2.5-4\,km/h$.     * Love (L) Waves: Near ground surface; rolling motion producing vertical movement; speed: $2-6\,km/h$.     * Rayleigh (R) Waves: Vertical and horizontal displacement; compressional; speed: $1-5\,km/h$.
• Destructiveness: Secondary and Love waves are the most destructive due to large amplitudes. Aftershocks are caused by the arrival of different wave types at different times.

Secondary Hazards and Tsunami Mechanics

Secondary Earthquake Hazards

• Soil Liquefaction: Affects poorly compacted sand and silt. Moisture separates from soil particles and rises, making the soil behave like a liquid, leading to building subsidence or landslides.
• Landslides: Shaking weakens cliff faces and hills. Unconsolidated material can travel several miles and accumulate mass. Risk is determined by topography, rainfall, and land use.

Tsunamis

• Generation: Usually generated in subduction zones at convergent margins. When oceanic crust jolts, the water column above is displaced upward. Gravity pulls it back, transferring energy into a wave.
• Physics: Fast travel with low amplitude in the open ocean. Near the coast, shallow water causes friction with the sea bed, slowing the wave and increasing its height into a wall of water (average $10\,feet$, up to $100\,feet$).
• Vulnerability Factors:     * Shape of Land: Bays funnel and concentrate waves.     * Physical factors: Wave amplitude, coastal gradient, population density, coastal defenses.     * Human factors: Warning systems, economic development, evacuation readiness.

Volcanic Hazards

Primary Hazards (Fast Onset)

• Lava Flows: Streams of erupted lava. Danger depends on viscosity (determined by silicon dioxide content).
• Pyroclastic Flows: Mixtures of hot dense rock, lava, ash, and gases moving at high speeds. Causes extreme danger and asphyxiation.
• Tephra and Ash Falls: Blasted volcanic rock and ash. Weight of ash can cause buildings to collapse.
• Volcanic Gases: Release of sulphur dioxide and carbon monoxide which can travel long distances.

Secondary Hazards

• Lahars: Fast-moving mudflows of rock, mud, and water. Triggered by melting snow/ice or heavy rain during eruptions.
• Jokulhlaup: Dangerous sudden floods caused by the melting of glaciers.
• Acid Rain: Caused by the atmospheric release of sulphur dioxide.

Models and Measures of Tectonic Events

• Hazard vs. Disaster:     * UNISDR Definition: A disaster is a serious disruption involving widespread losses that exceed a community's ability to cope using its own resources.     * Degg’s Model: Shows that a disaster only occurs when a vulnerable population is exposed to a hazard.
• Risk Equation: $\text{Risk} = \frac{\text{Hazard} \times \text{Vulnerability}}{\text{Capacity to Cope}}$
• Park Model (Disaster Response Curve):     * Stage 1 - Relief (hours-days): Search and rescue, immediate medical aid, appeal for foreign aid.     * Stage 2 - Rehabilitation (days-weeks): Restoration of services, temporary shelters, food/water distribution.     * Stage 3 - Reconstruction (weeks-years): Rebuilding infrastructure, restoring ecosystems, regrowing crops, mitigation for future events.
• Pressure and Release (PAR) Model: Analyzes vulnerability through a progression:     * Root Causes: Economic, demographic, or political processes (e.g., weak governance).     * Dynamic Pressures: Local factors (e.g., rapid urbanization, lack of training).     * Unsafe Conditions: Physical factors (e.g., dangerous locations, lack of clean water).
• Measurement Scales:     1. Volcanic Explosivity Index (VEI): Logarithmic scale ($0-8$) based on ejected material height and duration.     2. Modified Mercalli Scale: Subjective scale ($I$ to $XII$) based on perceived shaking and structural damage.     3. Moment Magnitude Scale: Measures energy released ($0-9$).     4. Richter Scale: Logarithmic scale measuring wave amplitude.

Management and Governance

• Hazard Management Cycle: Includes Preparedness, Response, Recovery, and Mitigation.
• Management Approaches:     * Modify the Event:         * Micro: Strengthening individual structures (e.g., building codes).         * Macro: Large-scale protection (e.g., Tsunami walls, planting mangroves to dissipate energy, lava diversion via water cooling).     * Modify the Vulnerability: High-tech monitoring, education, community preparedness, and adaptation (relocation).     * Modify the Loss: Short-term aid, long-term aid/reconstruction, and insurance.
• The Risk Poverty Nexus: Suggests poverty is both a cause and consequence of hazards, creating a positive feedback loop of loss.
• Mega-Disasters: Large-scale, rare, low-probability events affecting large areas/populations and often requiring international aid.     * Example 1: 2011 Tohoku Earthquake/Tsunami—disrupted global supply chains for Toyota and BMW.     * Example 2: 2011 Eyjafjallajökull eruption—ash cloud halted EU air trade, causing Kenyan flowers to wilt and lose revenue.